1. Field of the Invention
The present invention relates to coating powder for use for production of a high-temperature Ir-based alloy material or Ru-based alloy material having excellent high-temperature oxidation resistance, and to an alloy material formed with the powder which has excellent oxidation resistance characteristics.
2. Description of Related Art
Ir (iridium) or Ru (ruthenium) is a metal excellent in high-temperature strength and corrosion resistance and is expected to be used as a structural material for use in ultimate environments, and is used, for example, as crucibles for growth of single crystals for optical devices and as automobile engine plugs.
However, these metals form oxides IrO3 and RuO3 in a high-temperature oxidation atmosphere, and the sublimation points of these oxides are lower than the sublimation points of the metals, that is, it the sublimation point of IrO3 is 1092° C. and that of RuO3 is 1200° C. Accordingly, there is a problem in that Ir or Ru could not be used at the temperature of 1100° C. or higher in an oxidative atmosphere. As a method for solving the problem, a method of coating the metal surface with an Al2O3 (alumina) coating film of an oxidation-resistant material was employed. However, since the metal surface and alumina differ in point of the crystal structure and the lattice constant, the Al2O3 layer readily peel off from each other, and direct coating with alumina is ineffective as an oxidation-resistant coating film.
There has been tried a method of directly reacting Al (aluminum) with the surface of an Ir-based alloy or Ru-based alloy through aluminization. However, in reaction of Al with an Ir-based alloy or Ru-based alloy, molten Al explosively reacts with Ir or Ru at a temperature of the melting point of Al or higher, and therefore the formed film could not have a suitably controlled structure as an oxidation-resistant protective coating film. Given the situation, the present inventors have proposed a production method for a high-temperature material through aluminization of an Ir-based alloy or Ru-based alloy in WO2012/033160, in which an Fe—Al alloy powder or an Ni—Al composite powder is used for controlling the activity of Al.
In the production method proposed in WO2012/033160, IrAl or RuAl is formed on the surface of an Ir-based alloy or Ru-based alloy. However, later investigations have revealed that in case where an Fe—Al alloy powder is used, an Fe segregation phase is easy to form in the interface between the Ir-based alloy or Ru-based alloy and the IrAl film or RuAl film. Fe (ferrum or iron) rapidly diffuses in the Ir-based alloy or Ru-based alloy often to lower the alloy strength in use at high temperatures or often worsen the heat resistance of the IrAl film or RuAl film. Consequently, it is desirable to aluminize with no use of an Fe—Al alloy powder. However, an Ni—Al composite powder is poorly active since Ni is coated on the surface of the Al powder therein, and therefore the Ni—Al composite powder alone could hardly form an IrAl film or RuAl film. In addition, for suitably controlling the activity of Al, it is necessary to form an Ni—Al alloy powder in which the Al amount is controlled.